1. Field of the Invention
The present invention relates to a thermal roller fixing device for use in the fixing portion of electronic copying machines and the like.
2. Description of the Related Arts
Conventional thermal roller fixing devices for electronic copying machines provide a heating roller with an internal heater and comprising a conductive core member over which is superimposed a heat-resistant elastic layer formed of silicone rubber or the like, ad a pressure roller which makes pressure contact with said heating roller and comprises a heat-resistant elastic layer superimposed on a core member. In said fixing device, a toner image is fused onto a copy sheet by passing between the heating roller and the pressure roller with the toner image-bearing surface of said copy sheet disposed facing said heating roller.
In recent years, full color image forming apparatus having been developed for forming color images using four types of toner comprising black toner in addition to toners in three primary colors. In said apparatus, the four color toner images are superimposed one over another to form a single image which is then fixed onto the copy sheet to form a full color image. In order to form a full color image having excellent quality and gloss as well as transmission image color reproduction characteristics, a low viscosity toner must be used that readily adheres to the copy sheet and provides excellent transmittance. It is necessary to use a fixing roller that acts uniformly on the surface of the fused toner image so as to maintain the transmittance characteristics of the toner image fused to the copy sheet. In a full color image forming apparatus, therefore, the fixing roller is provided with a coating of a separation material on the surface of said roller to provide as smooth a roller surface as possible so as to act uniformly on the surface of the fused toner image while preventing the low viscosity toner from adhering to the surface of said fixing roller.
Image forming apparatus which form single images by a single toner image, on the other hand, typically provide a separation member on the heating roller side to reliably separate the copy sheet from the heating roller and allow steady transport of said copy sheet between the fixing rollers.
In contrast to the aforesaid image forming apparatus, a full color image forming apparatus which forms a single image by overlaying four color toner images frequently transports the copy sheet bearing a solid (beta) image so as to pass between the fixing rollers. Accordingly, the when a separation member is provided on the heating roller side of the thermal roller fixing device of a full color image forming apparatus, the surface of the copy sheet bearing the toner image comes into contact with said separation member with a high probability that the image quality will be adversely affected thereby. Furthermore, the surface of the fixing roller may be damaged through contact with the separation member, thereby producing defects in the image fused to the copy sheet. If a separation member is not used, however, the copy sheet tends to wrap around the heating roller. This disadvantage becomes particularly marked when the copy sheet used is a neutral paper sheet which contains calcium carbonate.
When the thickness of the heating roller is increased, the copy sheet tends to curl toward the pressure roller side so that the copy sheet separates from the heating roller without using a separation member. However, when the thickness of the heating roller is increased, it becomes difficult to transmit the heat from the internal heater within the roller and said heat accumulates between the metal core member and the heat-resistant elastic layer causing the deterioration of said heat resistant elastic layer.